1. Field of the Invention
This invention relates to a new method of making a positive slope at the step edge of layers being vacuum deposited to make Josephson junction devices or semiconductor devices. More particularly, the present invention provides a method of controlling the angle of positive slope step edge of vacuum deposited materials so that vertical steps and negative steps are eliminated and replaced by symmetrical gentle slope step changes.
2. Description of the Prior Art
Deposition of layers of metals and insulating materials by evaporation in a vacuum chamber are known techniques employed to make Josephson junction devices and semiconductor devices. Materials to be deposited are placed in a boat or a container and are heated until evaporation of the material occurs. In a high vacuum environment, the molecules and atoms of the vaporized material radiate outwardly from the container isotropically. The wafer or the substrate which is being processed is placed at a distance from the point source of the evaporating material. The atoms and molecules of the evaporated materials traverse the distance between the source container and the wafer in a straight line and are built up as a layer over the wafer as well as the surrounding equipment in the vacuum chamber.
When portions of a wafer are covered with a standard photoresist pattern, they restrict the deposition of the evaporated material to a predetermined area on the wafer. The photoresist pattern or stencil may form a vertical high step or vertical wall at the edge of the predetermined area or the wall may be tapered. Recent developments in making photoresist stencils have enabled the making of photoresist patterns which have tapered sides.
If a point source of evaporative material is directed on a straight line inside of the aperture formed by vertical walls of a photoresist pattern, the base area of the substrate may or may not be coated. When the point source of evaporating material strikes the top of the photoresist stencil on a line of sight which misses the vertical wall of the photoresist, the material will be deposited on one of the vertical walls and part of the material will be deposited so as to miss the other vertical wall of the photoresist pattern.
Photoresist patterns can be made which have a larger exposed area at the base on the wafer than at the top of the photoresist exposed to the evaporation of material. Thus, the photoresist pattern is undercut at the top surface and provides an offset ledge. Such photoresist stencils which have offset ledges are referred to as lift-off photoresist patterns. When evaporated material is deposited over such lift-off patterns, the top edge of the lift-off pattern defines an aperture window through which the evaporated material is projected along a straight line to be deposited on a projected area of the wafer exposed to the line of sight of the evaporated material. Thus, it is possible to generate on the same electrode or area being deposited a positive slope at one edge and a negative slope at the other edge.
When a small wafer is placed in a vacuum chamber at a substantial distance from the source of the evaporating material, the layer being deposited will have substantially vertical edge walls. If the wafer is large and/or the distance to the source of the evaporating material is small, there is a large angular deviation from the vertical or normal axis line of sight. When this deviation angle begins to exceed ten degrees, portions of the deposited material will have a positive slope at one edge and other portions at the opposite side of the wafer will have a negative slope.
Negative slope at the edge of deposited layers creates conditions which result in cracks in the insulation and voids at the undercut areas of the negative slope resulting in improper insulation and/or discontinuities between the subsequent layers deposited on top of the negative slope edge.
It has been proposed that large substrates and wafers may be mounted on articulating or rotary supports so that the portions of the wafer subject to the largest deviation angle is rotated closer to the normal axis during a portion of the deposition time. If any portion of the wafer encompasses the axis of rotation, there will still be angular deviation across the surface of the wafer. Rotating and evaporating at oblique angles tends to decrease the negative slope of deposited layers when small wafers are being processed, but it does not solve the negative slope problem when applied to large wafers.
When rotating and/or articulating wafers, the support means employed to provide this movement makes it extremely difficult, if not impossible, to cool the substrate to the very low temperatures that are often required in the manufacture of Josephson junction devices.
Accordingly, it would be desirable to deposit layers of metals or insulating materials by evaporation and deposition in a vacuum chamber in a manner which eliminates negative slope at the step edge of the deposited layers. Further, it would be desirable to provide a method of depositing layers of materials in a vacuum chamber which have symmetrical positive slopes without the requirement of moving the wafer.